Turbojet engine case, notably intermediate case

ABSTRACT

The present invention relates to an annular case ( 20 ) of a multi-flow turbojet engine comprising a first element ( 22 ) forming a hub, a second element ( 24 ) forming a cylindrical casing, radially on the outside of and concentric with the first element, radial arms ( 23 ) connecting the first element ( 22 ) to the second element ( 24 ), at least part of said arms being structural and having an aerodynamic flow straightener vane profile, characterized in that it comprises a first ring sector ( 20 C) of the case and a second ring sector ( 20 T), the first ring sector being made of composite material and the second ring sector of metal, said radial arms ( 23 T) of the second ring sector ( 20 T) being structural.

The present invention relates to the field of multi-flow turbojet engines and is aimed more particularly at a case element which is usually termed the intermediate case.

PRIOR ART

A multi-flow turbojet engine such as a front fan twin spool turbofan engine comprises an air intake fixed to a fan case itself bolted to the outer shroud of the case known as the intermediate case. The duct downstream of the fan communicates with two concentric ducts: the primary flow duct and the secondary or bypass flow duct. The primary flow duct leads to the compression stages and to the combustion chamber. The latter opens into the flowpath for the hot gases comprising the turbine wheels driving the compressors including the fan rotor. Following expansion, the gases of the primary flow are discharged via a central nozzle. The secondary flow duct which is annularly on the outside of the duct for the primary flow is straightened axially then passes between the arms of the intermediate case before being ejected through a secondary flow nozzle if the engine is of the type in which the flows are kept separate.

The rotary parts of a turbojet engine are guided in their rotation by rolling bearings generally supported by two case elements, one of them positioned at the front and forming the intermediate case and the other at the rear and forming the exhaust case. Furthermore, the transmission of load between the engine and the aircraft is performed by attachments secured to these two case elements.

The intermediate case is a component which may be of large diameter in so far as its external diameter is that of the fan. It comprises a hub through which the rotary shafts of the engine pass and which supports the bearings thereof, and through which the primary flow duct also passes. Structural arms extend radially from the hub as far as an outer cylindrical casing. The latter is formed of a shroud on which fittings are formed for attachment to a strut for suspending from an aircraft. Much of the load between the engine and the aircraft thus passes through this case.

On account of its structural role and of its size, the case makes a significant contribution toward the mass of the engine. This characteristic is all the more pronounced in the case of high bypass ratio engines in which the ratio between the cold secondary or bypass flow and the hot primary flow is high, of the order of 12 to 16, which are the target of development work on account of their low specific fuel consumption.

It would seem that the diameter of these components cannot be appreciably increased simply by extrapolating from known structures.

The problem is therefore that of finding an intermediate case structure that is light enough in weight that it can be incorporated notably by way of intermediate case in a high bypass ratio engine without impairing the specific consumption thereof.

According to one prior art, a bladed disk of fixed guide vanes is arranged in the secondary or bypass flow duct, the vanes being designed to straighten, before it is discharged into the atmosphere, the flow of air set in rotation about the engine axis by its passage through the fan. These vanes are often known by the abbreviation OGV, which stands for Outlet Guide Vanes. These vanes have only an aerodynamic role and are connected to the fan case by bolted connections. In this case the intermediate case is a one-piece metal component of all-welded construction. This solution is undesirable for high bypass ratio engines because the structural arms of the intermediate case introduce an aerodynamic pressure drop and thus impair the propulsion efficiency of the engine.

According to another prior art, the vanes that form flow straighteners are designed also to act as structural arms. They are connected by bolted connections to the outer shroud and to the hub of the case. However, despite the presence of the bolts, these vanes cannot be removed without the engine being dismantled from the wing structure of the airplane. The vanes are made of metal, as too are the shroud and the hub of the case. The disadvantage of this solution is that it adds the mass of the bolted connections. In addition, the flow path has to be rebuilt over the bolt heads in order to prevent the engine flow creating a drag effect.

One solution that improves on the previous one is described in Patent Application WO 2010/122053 in the name of the present applicant. The intermediate case comprises structural arms connecting between the hub and the outer shroud. These arms combine the mechanical function of transmitting load and the aerodynamic function. To do so they comprise, on the one hand, a plurality of metal ties extending radially along the length of the arms and, on the other hand, a shell made of composite material surrounding the ties and forming the aerodynamic exterior surface. However, the bolted connections are present again in this embodiment too.

Another solution might make it possible to dispense with the bolted connections of the previous solution and incorporate the straightening blades into the hub and the outer shroud. This would involve designing a one-piece case made of composite material which would have the additional advantage of saving weight. However, this solution would be complicated to produce and it would be difficult to guarantee a quality that was repeatable from one case to another. Further, if all the straightening vanes were made of composite a greater thickness would be required between the hub and the outer shroud. That would lead to a not-negligible pressure drop within the flow path.

SUMMARY OF THE INVENTION

It is an object of the invention to improve on the existing solutions in terms of weight and aerodynamic efficiency.

Thus, a subject of the invention is an annular case of a multi-flow turbojet engine comprising a first element forming a hub, a second element forming a cylindrical casing, radially on the outside of and concentric with the first element, radial arms connecting the first annular element to the second annular element, at least part of said arms being structural and having an aerodynamic flow straightener vane profile.

According to the invention, the annular case is characterized that it comprises a first ring sector and a second ring sector; the first ring sector is made at least partially of composite material and the second ring sector is made of metal, said arms of the second ring sector being structural.

More specifically, the first case ring sector has a resin impregnated fibrous structure and notably is of one piece with the arms that connect the annular elements configured to form flow straightening vanes and which are incorporated into the two elements, hub and cylindrical casing, of the first ring sector.

By means of the invention, most of the transmission of load is concentrated into the metal part of the second ring sector. Further, by making the first ring sector as one piece, the bolted connections are avoided, and this is advantageous in terms of mass.

According to another feature, the second ring sector is of one piece; more specifically, it is a casting notably made of titanium alloy.

According to another feature, the two ring sectors are joined together by bolting; more specifically they are joined together using fishplates.

According to another feature, the second element of the second ring sector comprises attachment means for fixing the turbojet engine to the structure of an aircraft. Advantageously, in the context of this application, the angle subtended at the center of the second ring sector is comprised between 30° and 120°.

The invention also relates to the turbojet engine that incorporates the new case by way of intermediate case.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages will become apparent from the following description of one nonlimiting embodiment given with reference to the attached drawings in which:

FIG. 1 is half of an axial section through the front part of a front fan twin spool turbofan engine, showing intermediate case elements;

FIG. 2 is a three quarters front perspective view of a case according to the invention, forming an intermediate case;

FIGS. 3 and 4 show the detail of the case of FIG. 2;

FIG. 5 is one step in the production of the first ring sector according to one mode of manufacture.

DETAILED DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION

FIG. 1 is taken from Patent Application WO2010/122053 which relates to an intermediate case structure with arms mechanically connecting the hub to the outer cylindrical casing. These arms combine the structural function of transmitting loads between the hub and the casing and the aerodynamic function of straightening the secondary or bypass flow. The present invention is aimed at a case structure that forms an intermediate case that is an improvement on the solution disclosed in that application.

Thus, the turbojet engine 1, of axis 3, in FIG. 1 comprises, in the general direction F in which the air flows from left to right in the figure, an air intake 2, a fan 4, a splitter 6, that splits the flow between a annular duct 8 for the primary flow F1 and an annular duct 10 for the secondary or bypass flow F2, radially on the outside of the primary flow duct 8. The primary flow F1 is compressed by compression means 7 which lead to the combustion chamber, not depicted, downstream. The fan is contained in a fan case 5 which delimits the duct 10 for the secondary or bypass flow F2 with the fairing of the primary flow duct. The fan duct 5 is bolted to the outer shroud 14 of the intermediate case 12. This case 12 comprises a hub 15 centered on the axis 3 of the engine and radial arms 13 mechanically connecting the hub 15 to the shroud 14 and uniformly distributed about the axis of the engine. The outer shroud 14 forms a cylindrical casing in aerodynamic continuation of the internal wall of the fan duct 5. As can be seen from the figure, the hub 15 comprises openings for the primary flow duct 8 and a central opening for the shafts of the rotors of the engine and their bearings.

In the structure set out in application WO2010/122053, each arm 13 comprises a metal core formed of ties bolted at their two ends to the hub and to the shroud and a shell made of composite material, the exterior surface of which is of an aerodynamic shape in order to act as a flow straightener.

The structure of the case of the invention is illustrated in FIGS. 2 to 4. The case 20, of annular shape about an axis that may coincide with that of the engine on which it is mounted, comprises a first element forming a hub 22 and a second element forming a cylindrical casing 24 leaving an annular space to the hub. Radial arms 23 mechanically connect the hub and the casing 24 and are circumferentially distributed about the axis of the case. These arms 23 have an aerodynamic profile which allows them to straighten an incident air flow rotating about the axis, i.e. which has an axial direction with a circumferential component. The flow of air that has passed through the annular space of the case finds itself substantially along the axis of the case further downstream. The number and profile of the arms are thus dictated by the flow straightener vane function conferred upon them.

According to the invention, the case here is made up of two ring sections centered on the axis of the case. The first ring section 20C is of one piece in as much as the arms 23C are incorporated into the parts of the first element forming the hub 22C and of the second element forming the casing 24C of the sector, thereby forming a single piece; the sector is also mainly made of composite material with a resin impregnated fiber structure. A metal coating may potentially be applied to the leading edge of the flow straightening vanes in order to increase their resistance to erosion and to impact. This leading edge coating is preferably added on in order to make it easier to replace in the event of damage.

The second ring sector 20T is made of metal. Advantageously, this sector is also of one piece. The radial arms 23T mechanically connecting the hub and the casing are incorporated into the parts of the first element forming a hub 22T and of the second element forming a casing 24T of the ring sector 20T. The ring sector is preferably obtained using the casting technique and is made of titanium alloy. The function of the arms 23T is twofold: aerodynamic and structural. Because of their aerodynamic profile, the arms 23T act as flow straightening vanes in the same way as the radial arms 23C. Because of their metal structure they transmit load between the hub and the casing.

The two sectors 20C and 20T are fitted together to form the case 20 and means of attachment between the two components may consist of fishplates 26 joining the second casing elements 24C and 24T together. The hub elements may also potentially be joined together. The fishplates are fixed for example by bolting.

When the case is used as an intermediate case, it comprises a means of attachment 28 on the casing element 24T. This means of attachment allows the engine to be suspended from an aircraft strut for example. It may be cast in with the remainder of the second ring sector. Through this contrivance, most of the load passes through the radial arms 23T.

To ensure this transmission of load between the hub and the casing, the angle subtended by the second ring sector is preferably comprised between 30° and 120° with respect to attachment 28, or even preferably between 15° and 60° on each side of the attachment.

Regarding the way in which the first ring sector made of composite material is manufactured.

One nonlimiting method of manufacture is to produce a fibrous structure using, for example, a three dimensional weaving technique using a weave of the interlock type covering several layers of warp threads and weft threads. One application of this three dimensional weaving technique is described in patent FR 2 913 053 in the name of the present applicant for the manufacture of a fan case.

In the present application, two 3D textile cloths are woven in such a way as to cause loops of warp thread to project out from the plane of weaving. These loops form protrusions that project out from the plane of weaving and will act as anchoring structures for the straightener vanes. The arrangement and spacing of the loops are thus determined by the vanes they are to accept.

FIG. 5 shows two cylindrical and concentric fibrous structures 122 c and 124 c which have been formed from the cloths to constitute the shrouds that form the first and second elements of the case. On the cloth of the interior cylinder 122 c, the protrusions 122 c′ formed by the loops face outward. On the exterior cylindrical cloth 124 c the protrusions 124 c′ face inward. The protrusions 122 c′ and 124 c′ are aligned in radial directions.

Fibrous structures 123, made up of fibers such as carbon fibers which are braided and in the form of sleeves, are then slipped around each pair of protrusions aligned on one and the same radius. They form connections between the interior cylindrical fibrous structure and the exterior cylindrical fibrous structure. These sleeves are shaped so that, after molding, they will form the straightening vanes. Once all the sleeves are in position on the cylindrical structures the entire entity is placed in a suitable mold, the volume of which corresponds to that of the shrouds, with the first and second elements 22 c and 24 c joined together by flow straightener vanes, and a resin is injected into the mold.

The component obtained is cut to form the first ring sector with which the second ring sector is then combined, the assembly constituting the composite case of the invention. In an alternative form, the ring sector is formed directly by molding.

The case structure of the invention thus allows a significant weight saving by comparison with the prior art, notably when applied to the intermediate case of a high bypass ratio engine. 

1. An annular case of a multi-flow turbojet engine comprising a first element forming a hub, a second element forming a cylindrical casing, radially on the outside of and concentric with the first element, radial arms connecting the first element to the second element, at least part of said arms being structural and having an aerodynamic flow straightener vane profile, said annular case further comprising a first ring sector of the case and a second ring sector, the first ring sector being made at least partially of composite material and the second ring sector of metal, said radial arms of the second ring sector being structural.
 2. The case according to claim 1, in which the first ring sector has a resin impregnated fibrous structure.
 3. The case according to claim 2, in which the first ring sector is of one piece and the radial arms of the first sector form flow straightener vanes.
 4. The case according to claim 3, in which the second ring sector is of one piece.
 5. The case according to claim 4, in which the second ring sector is a casting.
 6. The case according to claim 4, in which the second ring sector is made of titanium alloy.
 7. The case according to claim 6, in which the two ring sectors are joined together by bolting.
 8. The case according to claim 7, in which the two ring sectors are joined together using fishplates.
 9. The case according to claim 8, in which the second element of the second ring sector comprises attachment means for fixing the turbojet engine, on which the case is mounted, to the structure of an aircraft; the angle subtended at the center of the second ring sector being comprised between 30° and 120°.
 10. A multi-flow turbojet engine comprising an intermediate case at least partially incorporating a case according to claim
 1. 